Although the invention is of course not limited to any particular type of communication system comprising a packet data communication network or a circuit switched communication network and a packet switched communication network in combination, the background of the invention can most be easily understood by reference to the GPRS (General Packet Radio Service) system in GSM (Global System for Mobile Communication). GPRS is a new packet switched service which is standardized by ETSI.
As shown in FIG. 1, a communication system SYS incorporating a GPRS network architecture comprises for the conventional circuit switched mobile communication network the well-known entities of e.g. a mobile switching centre MSC, a base transceiver station BTS, a base station controller BSC, a home location register HLR etc. wherein the mobile switching MSC may be connected to a conventional public switched telephone network PSTN.
The GPRS architecture (illustrated with grey shading) introduces new packet switching functionalities in the existing GSM architecture. According to the GSM specifications a new node SGSN (SGSN: Serving GPRS Support Node) is provided which is interfaced via interfaces Gb, Gs, Gr with the base station controller BSC, the mobile switching centre MSC and the home location register HLR. Via the SGSN node an IP backbone network can be accessible in the conventional mobile communication network. By means of additional nodes GGSN (GGSN: Gateway GPRS Support Node) an IP network or X.25 network can for example be connected to the IP backbone network.
In FIG. 1 the dotted lines denote an exchange of signalling messages, the dashed lines denote a circuit switched connection and the solid lines denote a packet switched connection.
The existing GSM data services (9.6 k-bit/s packet switched) and a newly standardized High Speech Circuit Switched Data HSCSD and GPRS with data rates up to 114 k-bit/s are based on a Gaussian Minimum Shift Keying (GMSK) Modulation Scheme. To be able to have even higher bit rates a 8-Phase Shift-Keying (PSK) modulation scheme is introduced with an advanced standard, called the EDGE standard, which can boost the available data rate up to 384 k-bit/s (EDGE: enhanced Data Rate for GSM Evolution). A new extension called EGPRS (Enhanced General Packet Radio System) is considered as the migration from the second generation mobile network to the third generation Wideband Code Division Multiplex Access (WCDMA) networks. As shown in FIG. 1, GPRS provides a packet switched transmission service to transport data in a mobile environment. This packet oriented data service for GSM supports protocols like X.25 and IP as level 3 protocols and therefore is suitable to work as air link for the access to the IP based Internet. Another advantage in respect of Internet application in the mobile communication network via GPRS is that a packet oriented service no longer needs a costly online connection (i.e. an available online connection) applying time based charging but enables volume based charging.
In the system in FIG. 1 the aim is that the communication system SYS shall be able to support all existing applications via packet switched links, including voice and video but should also support application with bursty traffic, such as Internet applications whose bursty nature requires efficient multiplexing on the GSM time slot (TS). The idea is to build a unified network based on IP providing service flexibility and independence of applications and the network.
In particular due to the time critical nature of speech it is important to meet the tight quality of service requirements of real time traffic. For example, in real time applications as VoIP (Voice over Internet Protocol) over GPRS and EGPRS, the end-to-end delay time of the transfer of data packets is an important aspect, since for example a high delay time might sound like a speech pause at the receiving end. Therefore, in particular for real time applications special provisions regarding the maximum delay time must be made.
Protocol Structure
In FIG. 2 the GPRS protocol structure for the communication network SYS in FIG. 1 is shown. MS is the protocol stack of the mobile station or subscriber terminal, more generally of a communication station. BSS is the base station system and SGSN and GGSN are the same nodes as explained above with reference to FIG. 1. It should be noted that a full description of this protocol structure is contained in the ETSI standard GSM 3.60 and hereinafter only those portions of the protocol structure are explained which are relevant for the present invention.
As shown in FIG. 2 the medium access layer MAC and the radio link layer RLC operate above the physical link layer. The MAC layer provides the multiplexing of several mobile stations MS on the time slot structure of GSM. The MAC layer arbitrates multiple mobile stations attempting to allocate resources and transmitting simultaneously. The operations of the MAC functions allow a single mobile station to use more than one time slot (TS) simultaneously. The number of allowed parallelly used time slots TS in a single TDMA frame is determined by the time slot capabilities of the mobile station. Hereinafter, these capabilities are called “multislot capability”. Each mobile station is thus given a certain amount of time slots TS for use, ranging from multislot capability 1 (only 1 time slot TS) up to multislot capability 8 (all 8 time slots TS in the TDMA frame).
The GRPRS MAC layer is responsible for providing efficient multiplexing of data and control signalling on the uplink and downlink connections. The multiplexing on the downlink is controlled by so-called downlink scheduler which has knowledge of the active mobile stations in the system and of the downlink traffic. Therefore, an efficient multiplexing on the timeslots TS can be made. On the uplink, the multiplexing is controlled by medium allocation to individual users. This is done by resource requests, which are sent by the mobile station to the network which then has to schedule the time slot TS on the uplink.
The GPRS RLC function provides the interface towards the LLC (LLC: Logical Link Control) layer, especially the segmentation and re-assembly of LLC-PDUs (PDU: Packet Data Units) into RLC data blocks depending on the used coding scheme (CS).
The procedures of the medium access layer MAC in the mobile station (communication station) on the terminal side and the base station system BSS on the network side NS include the provision of a physical connection which is called the Temporary Block Flow TBF in GPRS. A temporary Block Flow (TBF) is a physical connection used by the two RR peer entities to support the unidirectional transfer of LLC packet data units (PDUs) on packet data physical channels. The TBF is allocated radio resources on one or more packet data channels PDCHs and comprises a number of RLC/MAC blocks carrying one or more LLC PDUs. A TBF is temporary and is maintained only for the duration of the data transfer (i.e. until there are no more RLC/MAC blocks to be transmitted and in RLC acknowledgement mode, all of the transmitted RLC/MAC blocks have been successfully acknowledged by the receiving entity). The physical connection TBF is assigned a temporary flow identifier (TFI) by the network side NS to associate the mobile station MS with the current physical connection TBF.
For example, an uplink state flag (USF) is used by the network side NS (i.e. the network scheduler) to control the multiplexing of the different mobile stations on the uplink connection (for the packet transfer). The uplink state flag USF is included in the header of each RLC PDU packet on the downlink connection (Packet Data Channel PDCH). The uplink state flag USF indicates the owner of the corresponding uplink data packet (radio block). The mobile station MS which has the identity indicated in the USF field is allowed to transmit a RLC block (data packet) in the uplink direction on the same time slot TS on which it has received the radio block with the corresponding uplink state flag USF.
Thus, the physical connection is used to organize the access of the radio resources. A mobile station MS having a valid TBF is therefore included in the GPRS scheduling mechanism and can expect to get access to the radio resources according to its signalled multislot capabilities. Thus, the physical connection indicates in the subscriber terminal (mobile station) and in the network side (base station system BSS) that the subscriber terminal and the network side are valid for performing a packet data transfer. Via this physical connection the subscriber terminal side and the network side know that the subscriber terminal (mobile station or communication station) should be included in the GPRS timeslot (radio resources) scheduling. Thus, via the physical connection a context is generated in the subscriber terminal side and the network side which indicates the subscriber terminal and network side as being included in the packet data communication system radio resources scheduling process. This context or physical connection is only maintained during the data packet transfer and is terminated as soon as a packet data transfer stops.
Real Time Application (Voice Coder)
There are applications like real-time applications, which are sensitive against delays occurring during the end-to-end data packet transfer. In particular, this applies to voice coding (a real time application), without being limited to it.
With increasing processing power it became beneficial to compress voice/audio information before sending it to the subscriber terminal or the network side. This is especially true for transmission of speech/audio over wireless channels because transmission costs are much higher than computing costs in this environment. Nowadays, many different coders have been employed and are in use. Most of these coders generate a constant bit rate traffic (CBR) and produce data packets at typical and well defined regular intervals. The coder standard G.723.1 may serve as a typical example of the coders. Data packets containing compressed speech information are produced with inter-arrival times TDIFF of 30 ms and the data packets are typically 24 bytes in size.
A coder on the transmitting subscriber terminal side SS or the network side NS may use a silence detector to avoid generating packets during speech pauses. When the silence detector detects a silence period it sends a silence insertion descriptor SID as shown in FIG. 4b in order to indicate the silence period. In the silence period no data packets are generated. The silence insertion descriptor SID is also used to define the comfort noise level generated at the receiver site during the silence period. FIG. 4b shows a typical packet stream produced by such a coder according to G.723.1.
Of course, it depends on the coding standard used whether or not a silence insertion descriptor SID is send by the coder. That is, other coders may prefer not to insert a silence insertion descriptor in which case the silence periods are indicated to the receiver site differently.
In principle, the typical traffic shape shown in FIG. 4b can be generated by a coder or any real time application RTA connected or incorporated into the mobile station as shown in FIG. 3.
Transmission Queue TR-QUE
The data packets as generated in FIG. 4a by a real time coder for speech (or in fact by any other application connected to or incorporated into the mobile station MS) is transmitted by the subscriber terminal side or the network side from a transmission buffer containing a transmission queue TR-QUE illustrated in FIG. 3. As shown in FIG. 4a, the data packets DP1, DP2, DP3 . . . DPn are successively transmitted to the network side or subscriber terminal side from this transmitter queue TR-QUE. However, when transmitting encoded speech data packets/audio data packets over GPRS/EGPRS there is a certain threat that the systems behaves poorly due to the frequent and unnecessary releases of the physical connection TBF, even during active periods of a speaker. The inventors have discovered such a problem during their studies of experimental systems and simulations.
A reason for the frequent release of the physical connection TBF is the behaviour of GPRS focussing on a transmission of large application packet data units PDUs such as complete web-pages or simply the content of a TCP window (TCP: Transfer Control Protocol). For such applications which quickly and continuously generate data packets, the transmitter queue TR-QUE is likely to be filled and the individual data packets are successively transmitted whilst the physical connection TBF is not interrupted. In contrast to that, in the case of audio/speech transmission over (E)GPRS the transmitter queue TR-QUE is still constantly filled with small data packets from the application (the speech coder). For the case of the G.723.1 standard speech coder, an application packet enters the (E)GPRS transmitter queue TR-QUE every 30 ms. That is, for such a coder the inter-arrival time is typically 30 ms.
However, if the packet is transmitted from the queue in a shorter time than 30 ms, the transmitter queue TR-QUE is emptied (e.g. the queue shown in FIG. 4c is emptied) and in such a case the GPRS physical connection release procedures as shown in FIGS. 5a, 5b are immediately started. This leads especially for high-end terminals (high multislot capability) to the unwanted effect of frequent physical connection releases and establishments. In such a case the application and end-user would experience an unnecessary high end-to-end delay and furthermore, of course the repeated release and establishment of the physical connection TBF entails a heavy signalling load during the TBF handling.
FIG. 6 shows the end-to-end delay [ms] when different numbers of mobile stations MS simultaneously transmit packet data in the communication system SYS. As designated with the curves 8MS, 9MS, 10MS, 11MS there is a large end-to-end delay for prior art solutions. The inventors have discovered the problem that this high-end-to-end delay during the data packet transmission is due to frequent TBF releases. Since the TBF releases have been recognized as the core problem of the invention, hereinafter with reference to FIGS. 5a, 5b and FIG. 4c the procedure for uplink and downlink TBF release will be explained with more detail. It should also be noted that of course these release and establishment procedures for a physical connection are by no means limited to the real time application data packet patterns since a TBF release will start whenever an empty queue in the transmitter is detected, independent from the fact whether the data packets are generated by a real time application or any other application.
Physical Connection Release
As explained above, the establishment of physical connection TBF is done by using the signalling channels of GPRS. This means that a demand for a physical connection TBF needs to be signalled in the worst case on the random access channel. In general, the establishment of a physical connection TBF takes a certain time and occupies a signalling capacity in the communication system. The GPRS standard does not define exactly the conditions when a physical connection TBF has to be established and released. However, the method to perform the establishment and release procedures have been defined quite clearly.
Thus, with reference to FIG. 5a and FIG. 3 the release procedure for an uplink physical connection TBF is described. The subscriber terminal side comprises a subscriber terminal side transmitter queue monitoring device QUE-MON for determining whether the transmitter queue TR-QUE comprises data packets DP to be transmitted (see FIG. 4a). Furthermore, the subscriber terminal side comprises a transmitter queue information setting means CV-SET for determining on the basis of the determination made by the transmitter queue monitoring means QUE-MON a transmitter queue information CV indicating whether the transmitter queue is empty (CV=0) or whether the transmitter queue TR-QUE contains at least one remaining data packet to be transmitted to the network side (CV>0). The subscriber terminal side transmitter SS-TR transmits to the network side NS data packets DP from the transmitter queue TR-QUE and transmits in association with the respective data packet DP the determined transmitter queue information CV. The transmitter queue information CV can be transmitted in the respective packet DP as shown in FIG. 5a and FIG. 4c. However, it is of course sufficient to link the transmission of the data packet to the transmission of the respective transmitter queue information CV. Thus, every RLC/MAC data block sent from the subscriber terminal side to the network side contains the transmitter queue information CV (which hereinafter will also be referred to as the counter value CV field). Usually this counter value field CV is transmitted in the header and is used to signal the number of remaining RLC packets in the transmitter queue TR-QUE. FIG. 4c shows one example of the usage of the counter value CV field for a mobile station handling 2 time slots in a TDMA frame. As can be seen from FIG. 4c, for each transferred data packet a respective counter value field CV is determined, i.e. in FIG. 4c CV=2 for the first data packet (PDU) and CV=1 for the second packet (PDU)).
According to the ETSI standard GSM 04.60 V8.2.0 standard the transmitter terminal side transmitter queue information setting means CV-SET sets as said transmitter queue information CV a counter value CV determined in accordance with the following expression:Integer x=roundup ((TBC−BSN′−1)/NTS)CV=x, if x<=BS_CV_MAX15, otherwisewhere:    CV: counter value inserted in each data packet DP before transmission;    TBC: total number of data packets DP present in the transmitter queue TR-QUE;    NTS: transmission resources RES defined as a number of time slots (multislot capability NTS) in a single frame used for data packets DP transferred on the uplink connection with range 1 to 8;    BSN′: absolute block sequence number of the RLC data block with range from 0 to (TBC-1);    BS_CV_MAX: a parameter broadcast in the system information; and    roundup: rounds upwards to the nearest integer.
According to the standard, once a mobile station MS transmits a value of CV other than 15, the mobile station shall transmit exactly (TBC-BSN′−1) not transmitted RLC data blocks. In other words, a countdown procedure is started, which leads to the release of the physical connection TBF. In particular, in context with real-time applications, this can cause an unnecessary release of the physical connection TBF and therefore can introduce an unnecessary delay. Any data that arrives from the higher layer after the commencement of the countdown process shall be sent within a future physical countdown TBF.
Also without focusing on the countdown procedure, the normal resource assignment results in an unnecessary physical connection TBF release as shown in FIG. 4c. The transmitter queue information setting means CV-SET always determines at a certain time the number of data packets which remain when the present data packet is transmitted to the network side. Since for example in step ST4c1 the network side transmission resource scheduler SCH-RES had assigned two time slots 2TS (because the mobile station is a multislot capability 2 mobile station) the first data packet transferred in step ST4c2 receives a counter value CV=2 (CV=roundup [(4−1)/2]=roundup [1.5]=2). Likewise, the second data packet receives a counter value of CV=1 (CV=roundup [(3−1)/2]=roundup [0.5]=1.0. The assignment of two timeslots and the transmission of data slots with the respectively calculated counter value CV is continued in FIG. 4c in steps ST4c3, ST4c4, ST4c5 and ST4c6. In FIG. 4c a multislot capability 2 mobile station and an application generating a new data packet every 30 ms was assumed. However, also for the general case the calculation of the counter value CV and the transmission of the data packets is the same. That is, in a multislot capability×transmission maximum×timeslots are used for transmission as assigned beforehand by the network side NS and each of the x data packets have a corresponding counter value CV.
Furthermore, it should be noted that of course the transmission of the data packets by using timeslots can also be different. For example, each data packet can be distributed over the plurality of timeslots and can be reassembled on the network side NS. Still, after reassembly in the network side NS the respective counter value CV will indicate whether there are any further packets in the transmitter queue TR-QUE or not.
As shown in FIG. 5a, for the release of an uplink physical connection TBF, at a certain stage in step ST5a1 a RLC/MAC data packet containing a counter value CV=0 will be transmitted to the network side. The counter value CV=0 in a packet clearly indicates an empty queue to the network side after transmission, i.e. CV=0 indicates that there are no further “remaining” data packets in the queue after the transmission of the data packet containing CV=0. In this case the network side will first transmit a so-called packet uplink acknowledgement/negative acknowledgement message in step ST5a2 incorporating a final acknowledgement indicator=1 to the subscriber terminal side. The message in step ST5a2 is to indicate to the subscriber terminal side that the network side has understood that no further data packets are residing in the subscriber terminal side transmitter queue and that an uplink TBF release procedure is to be started. In step ST5a3 the mobile side sends a packet control acknowledgement message to the network side after releasing the physical connection TBF on the mobile side. Finally, after receiving the message in step ST5a3 the network side performs the release of the physical connection on the network side. As can clearly be seen, a certain time is needed to release or terminate the physical connection for the uplink and furthermore signalling resources are occupied in the network.
FIG. 5b shows the steps for the release of a downlink physical connection TBF. The procedure of a downlink physical connection release in FIG. 5b is also indicated with steps ST5a2, ST5a3 in FIG. 4c. As can be seen from FIG. 3, also the network side has a network side transmitter queue TR-QUE, a network side transmitter queue monitoring device QUE-MON, a network side transmitter queue information setting means FBI-SET and a network side transmitter NS-TR performing the same functions as the corresponding devices in the mobile station MS. However, the network side does not indicate to the terminal side the exact number of remaining data packets, i.e. the network side transmitter queue information setting means only determines a transmitter queue information FBI which indicates whether the transmitter queue TR-QUE is empty, FBI=1 or whether the transmitter queue TR-QUE contains at least one data packet to be transmitted to the terminal side, FBI=1. When the subscriber terminal side receives the message in step ST5b1 containing the final block indicator field FBI=1, then this indicates the occurrence of the last/final block of the current physical connection TBF. After successful reception of this RLC packet with FBI=1, the mobile side performs the physical connection release and sends an acknowledgement message to the network side in step ST5b2. Then the network side performs the release of the physical connection. As can be seen from FIG. 5b, also for the release of the downlink physical connection TBF time is necessary and signalling resources are used.